I use both a PDF in iBooks and also a PDF converted to individual images added to a album in Photos. I actually prefer the photo way as you can pinch to zoom out to go high level and dive back into any page. Useful if you have a longer, less linear portfolio. PDF in iBooks though is better to zoom into vector art and has the slick page turning animations.

There's also a pretty decent Behance app if you have your portfolio on there. I have it but have never really used it.

I really like Padfolios. Really nice clean program. You just up load you PDF files and it let's you organize them in into different groups. Which is great when tailoring a portfolio to different people.

Showing a portfolio on an iPad is great if you are doing it one-on-one, or if in a group setting, you have a connector to plug into a larger display/monitor, or use Airplay on an Apple TV enabled display.

Personally I think unless you are showing some interaction or iOS function that can only be seen on an iPad, the constant finger swiping (not to mention fingerprints) and limited viewing angle of the iPad is a distraction for your audience to absorb your information.

I personally use Minimal Folio. I love how I can swipe vertically to view more images of the same project or swipe horizontally to move to another project. This gives me more control over the pace of the presentation.
It has dropbox integration as well which makes it really easy to upload and sync.

However, for interviews it's easier for me to have a PDF saved to the bookshelf and a photo gallery folder with some hero shots of work that didn't make it to the portfolio, or detail/process shots. I find everyone is pretty familiar with the swipe and pinch interaction, and it's always pretty quick to pull up.